1. Field of the Invention
The present invention relates to a vibration wave driven motor for supplying electrical energy to an electro-mechanical energy conversion element, such as a piezo-electric element arranged on an elastic member, to vibrate a vibration member constituted by the elastic member and the conversion element, and to cause it to perform circular or elliptic motion about the mass point of the vibration member, thereby frictionally driving a moving member pressed against the vibration member and, more particularly, to a structure of the vibration wave driven motor.
2. Related Background Art
In a conventional vibration wave driven motor, a motor of a type which causes flexural vibration in a ring-like vibration member which is driven with a moving member frictionally engaged with the vibration member relative to each other is put into practical applications, e.g., an AF mechanism for a camera. However, since the motor has a ring-like structure, the cost of a unit including a compression mechanism is unexpectedly high. Thus, such a motor is disadvantageous in terms of cost in applications of motors, which do not require a ring-like structure, or in other words, a hollow structure. For this reason, a rod-like vibration wave driven motor which is of a solid type, and has a simple arrangement of, e.g., a compression system, as shown in FIGS. 4 and 5, has been recently proposed.
The conventional rod-like vibration wave driven motor will be briefly described below.
In FIGS. 4 and 5, a hollow upper vibration member 1 comprises a round metal rod constituted by forming a cone-shaped horn portion 1c between a small-diameter shaft portion la as a distal end portion, and a large-diameter shaft portion 1b as a rear end portion. A threaded portion 1d is formed in the inner circumferential surface of an axial hole of the member 1. A lower vibration member 2 comprises a round metal rod formed to have the same outer diameter as the large-diameter shaft portion 1b of the vibration member 1. A bolt insertion hole 2a is formed to coincide with the axis of the vibration member 2. Each of two ring-like piezo-electric element disks 3 and 4 is formed to have the same outer diameter as that of the large-diameter shaft portion lb. These disks 3 and 4 are arranged between the vibration members 1 and 2 via an electrode disk (not shown). Each of these piezo-electric element disks 3 and 4 is divided into two regions, and these two regions are polarized in different polarities in the direction of thickness. A bolt 6 fastens the vibration members 1 and 2, and is threadably engaged with the threaded portion 1d of the vibration member 1 via the bolt insertion hole 2a of the vibration member 2, thereby clamping and fixing the piezo-electric element disks 3 and 4 between the lower and upper vibration members 2 and 1. One piezo-electric element disk 3 is positionally offset from the other piezo-electric element disk 4 by 90.degree., and these disks 3 and 4 are arranged in the same direction. The two-divided electrode surface of one piezo-electric element disk 3 opposes the rear end face of the vibration member 1, and the electrode surface of the other piezo-electric element disk 4 opposes the common electrode surface of the piezo-electric element disk 3 via the electrode disk (not shown). The common electrode surface of the other piezo-electric element disk 4 is in contact with the front end face of the lower vibration member 2. When AC voltages V.sub.1 and V.sub.2 are applied across the two-end portions of the piezo-electric element disks 3 and 4 via the electrode disk, a vibration caused by an expansion/contraction displacement in the direction of thickness of the piezo-electric element disk 3, and a vibration caused by an expansion/contraction displacement in the direction of thickness of the piezo-electric element disk 4 occur. Upon composition of these vibrations, a rod-like vibration member A constituted by the vibration members 1 and 2, and the piezo-electric element disks 3 and 4 is vibrated.
The AC voltages V.sub.1 and V.sub.2 have the same amplitude and frequency, and have a 90.degree. time phase difference. The piezo-electric element disks 3 and 4 are arranged to be positionally offset by 90.degree. from each other.
Therefore, the vibration member A performs a circular motion around its axis like a skipping rope. When the phases of the voltages V.sub.1 and V.sub.2 are inverted, the rotation of the circular motion is reversed. Note that the principle of causing the circular or elliptic motion is known to those who are skilled in the art in, e.g., Japanese Patent Appln. Laid-Open No. 62-141980, and a detailed description thereof will be omitted.
In this case, a vibration mode is set such that the loop of the vibration is located at a predetermined position on the horn portion 1c. A rotor R is rotated by frictional contact between the distal end portion of the rotor R frictionally engaged with the distal end portion of the upper vibration member 1, and an antinode portion of the vibration formed in the horn portion 1c. A spring 5 biases the rotor R against the vibration member 1, and is looped between the distal end portion of the bolt 6 and the distal end portion of a hooking pin 7. The hooking pin 7 is mounted on an inner race portion of a thrust bearing 8 attached to one end portion of the rotor R, and applies the biasing force of the spring 5 to the rotor R.
However, in the conventional rod-like vibration wave driven motor, when the piezo-electric element disks 3 and 4 are fastened between the two vibration members 1 and 2 by the bolt 6, the following state occurs, as shown in FIG. 6. That is, a compression stress distribution in a direction perpendicular to the axis of the vibration member A is not uniform, in other words, a strain in the axial direction of the vibration member A in the piezo-electric member disks 3 and 4 is large at the inner periphery side, and is small at the outer periphery side.
Such a state occurs for the following reason. That is, when the piezo-electric element disks 3 and 4 are clamped, the upper and lower vibration members 1 and 2 are elastically deformed, and a stress is concentrated on the inner periphery side near the threaded portion of the bolt 6, in particular, on the inner periphery side of the upper vibration member 1.
For this reason, when the piezo-electric element disks 3 and 4 are clamped by fastening the bolt 6, the piezo-electric element disks 3 and 4 consisting of a piezo-electric ceramic easily crack, and the piezo-electric element disks 3 and 4 cannot be in uniform contact with the vibration members 1 and 2. As a result, vibration characteristics easily vary, resulting in low efficiency of the motor.